Current events underscore the need for an improved and inexpensive analytical device capable of rapidly, reliably, and accurately detecting explosives, toxic chemicals, biological agents, chemical warfare agents, and other harmful materials. Spectrometers based on ion mobility have been previously developed to serve this purpose, but technological improvements are still needed to reduce detection time, increase sensitivity, enable environment adaptability, reduce noise interference, improve prediction accuracy, and reduce power consumption.
Conventional spectrometers typically employ either ion-mobility spectrometry (IMS) or differential ion mobility spectrometry (DMS) as the broad method by which they identify compounds in a sample gas taken from an ambient environment. Conventional IMS devices, which are well known in the art, are based on time-of-flight (TOF-IMS) analysis. TOF-IMS identifies compounds by measuring the time it takes ions to travel through a drift tube, usually on the order of milliseconds, from a shutter-gate to a detector electrode. The drift time is dependent on the mobility of the ions in a linear, low electric field, which accelerates the ions in the drift tube. The measured drift time is characteristic of the ion species present in the sample. In IMS systems, an ion's mobility coefficient is independent of the electric field strength but its velocity is proportional to the electric field strength.
DMS devices operate by characterizing chemical substances using differences in the gas phase mobilities of ions in alternating, high-frequency, asymmetric electric fields. Ions are separated as they are carried by drift gas between two-parallel plates or filter electrodes. At higher electric field strengths there is a nonlinear dependence on ion mobility. A high-frequency asymmetric electric field is produced by applying a high-frequency asymmetric differential potential between the plates. An equivalent field could be produced by applying a differential potential to both plates relative to ground, or to one of the plates with the other grounded. This applied field, referred to as the separation or dispersion voltage, causes ions to oscillate perpendicular to the gas flow. Some ions traverse the filter electrodes, while others gradually move towards one of the electrodes and eventually collide with an electrode, which neutralize the electric charge in such ions. Only ions with a net velocity or differential mobility of zero transverse to the applied electric field will pass through the electrodes.
Despite the differences between various IMS and DMS systems, each system may share some of the same components. Both IMS and DMS based devices may use similar inlet systems to control the flow of chemical vapor into the system and to filter water vapor and other molecules that inhibit the ability of systems to effectively detect certain chemical vapors in a gas.
These inlet systems frequently employ gas-permeable membranes to selectively block certain molecules from passing into the IMS or DMS systems. But these inlet systems have disadvantages—a set dynamic range, restrictive flow, chemical-specific limitations.